1. Field of the Invention
The present invention relates to a method for separating certain hydrocarbons from a mixtures of hydrocarbons. More specifically, the present invention relates to a method for separating and concentrating specific hydrocarbons from a mixture of hydrocarbons. This is a problem commonly found in the petroleum refining industry or petrochemical industry.
2. Description of the Prior Art
In the petroleum refining industry and the petrochemical industry, methods for separating certain hydrocarbons from mixtures of hydrocarbons by using a separating membrane have been under scientific and economical development for many years. To this date, many approaches have been published. For example, U.S. Pat. No. 2,958,656 proposes a method wherein a mixture of hydrocarbons (that is, naphtha) is supplied to a non-porous cellulose ether membrane, through which one portion of the hydrocarbons is permeated. Subsequently, the permeated hydrocarbons are removed using a washing gas or a washing liquid, thereby separating unsaturated compounds, saturated compounds and aromatic compounds. U.S. Pat. No. 2,930,754 proposes a method wherein one portion of the mixture, having a distillate temperature approximately equal to the boiling point of gasoline, is selectively permeated through a non-porous cellulose ether membrane. The permeated hydrocarbons are then removed using a washing gas or a washing liquid, thereby separating unsaturated hydrocarbons and other hydrocarbons such as aromatic compounds. Many fluorine-containing polyimides are known to be materials for membrane separation with excellent heat resistance and gas permeatability. Aromatic polyimides containing fluorine are disclosed, for example, in Publication of Unexamined Japanese Patent Application (Tokkai) No. Hei 5-7749, U.S. Pat. No. 3,822,202, U.S. Pat. No. 3,899,309, U.S. Pat. No. 4,532,041, U.S. Pat. No. 4,645,824, U.S. Pat. No. 4,705,540, U.S. Pat. No. 4,717,393, U.S. Pat. No. 4,717,394, U.S. Pat. No. 4,838,900, U.S. Pat. No. 4,897,092, U.S. Pat. No. 4,932,982, U.S. Pat. No. 4,929,405, U.S. Pat. No. 4,981,497, and U.S. Pat. No. 5,042,992.
However, these conventional methods have an insufficient resistance of most of the separating membranes against aromatic hydrocarbons, unsaturated hydrocarbons, saturated hydrocarbons, etc. and have difficulties isolating specific hydrocarbons. Therefore, their use on an industrial scale is severely limited because of poor performance and high costs.